I live in an area that is a spawning ground for horseshoe crabs in early summer. They’re odd-looking creatures that remind me of miniature amphibious tanks storming the beach. I noticed two lateral eyes near the front of the dorsal shell that didn’t seem to be particularly useful when the crabs took circuitous routes to find their way back to the water. Curious as to how they navigated, I did some research and found some surprising information.

First, horseshoe crabs aren’t crabs at all.  Rather, they are arachnids, the same class as spiders and scorpions. What’s more, they have ten eyes! The two lateral eyes are compound eyes, meaning they consist of about 1,000 units, each consisting of a cornea, lens, and cones and rods. (Sound familiar?) These eyes are sensitive to polarized light and can magnify sunlight ten times and are a million times more sensitive to light at night than during the day. Two simple eyes that can sense ultraviolet light from the moon are forward and central to the lateral eyes. Behind the lateral eyes are two primitive eyes and three more along a ridge on the underside of the shell.  (Are you counting?) Photoreceptors on the telson (tail) make up the last eye. These photoreceptors are thought to help the horseshoe crab’s brain synchronize to light and dark cycles. With all those eyes and cones and rods the largest of any known animal, 100 times the size of those in humans, their vision is quite good. 

Horseshoe crabs have evolved very little in more than 200 million years, so they must be doing something right. But what can they tell us about our own eyes? In 1969, H. K. Hartline shared the Nobel Prize in Medicine or Physiology for introducing the concept of lateral inhibition through his research on horseshoe crab eyes. Lateral inhibition is the ability of excited neurons to reduce the activity of neighboring neurons. Lateral inhibition functions in visual perception by increasing the contrast and resolution of visual stimuli. Horseshoe crabs have been found to have 18 opsins, a photosensitive pigment in the cones. Genomes provide us with valuable information, as they contain the complete set of opsins for a particular organism. For example, humans possess nine different opsins. Three opsins are expressed in cone photoreceptor cells, which determine the three colors in our vision: red, green, and blue. A rhodopsin, which functions under dim light conditions, is expressed in rod photoreceptor cells. Melanopsin is the opsin that functions in the circadian regulatory system and pupil constriction of the eyes. In addition to these, we have encephalopsin, neuropsin, RGR opsin and peropsin. 

For more than fifty years, researchers have studied the horseshoe crab’s optic nerves, some of which are sensitive to light at 535 nanometers (green), and others to 380 nm (HEV). With three different types of eyes, large retinal neurons and well-defined visual behavior, the visual system of horseshoe crabs offers a wealth of information. What’s more, the structure and function of their eyes are regulated on a daily basis by a circadian clock in the animal’s brain. The eye-brain connection of horseshoe crabs is easily available to study and its unique functions have lent a great deal to the study of human vision.

Learn more about human eye development and function with our CE, Check Your Eye Development IQ, at 2020mag.com/ce.